Q.The decomposition of phosphine ($PH_3$) on tungsten at low pressure is a first order reaction. It is because the (NEET)

Step 1. Decomposition of $PH_3$ on hot tungsten is a surface-catalysed (heterogeneous) reaction -- $PH_3$ must first adsorb onto the tungsten surface before it decomposes, so the rate genuinely depends on how much of the surface is COVERED by adsorbed $PH_3$, not directly on gas-phase concentration.

Step 2. At LOW pressure, the surface is far from saturated, so the fraction of surface covered ($\theta$) is directly proportional to the gas pressure P: $\theta\propto P$. …

Recall the Langmuir-adsorption explanation for surface-catalysed decompositions: at low pressure, surface coverage (and hence rate) scales l …

• Choosing (c) 'rate is independent of surface coverage' -- that describes the HIGH-pressure limit (a saturated surface, giving zero order), the oppo …